From hidden order to antiferromagnetism: Electronic structure changes in Fe-doped URu2Si2

In matter, any spontaneous symmetry breaking induces a phase transition characterized by an order parameter, such as the magnetization vector in ferromagnets, or a macroscopic many-electron wave function in superconductors. Phase transitions with unknown order parameter are rare but extremely appealing, as they may lead to novel physics. An emblematic and still unsolved example is the transition of the heavy fermion compound URu2Si2 (URS) into the so-called hidden-order (HO) phase when the temperature drops below T0=17.5 K. Here, we show that the interaction between the heavy fermion and the conduction band states near the Fermi level has a key role in the emergence of the HO phase. Using angle-resolved photoemission spectroscopy, we find that while the Fermi surfaces of the HO and of a neighboring antiferromagnetic (AFM) phase of well-defined order parameter have the same topography, they differ in the size of some, but not all, of their electron pockets. Such a nonrigid change of the electronic structure indicates that a change in the interaction strength between states near the Fermi level is a crucial ingredient for the HO to AFM phase transition.

Hidden order across online extremist movements can be disrupted by nudging collective chemistry

Disrupting the emergence and evolution of potentially violent online extremist movements is a crucial challenge. Extremism research has analyzed such movements in detail, focusing on individual- and movement-level characteristics. But are there system-level commonalities in the ways these movements emerge and grow? Here we compare the growth of the Boogaloos, a new and increasingly prominent U.S. extremist movement, to the growth of online support for ISIS, a militant, terrorist organization based in the Middle East that follows a radical version of Islam. We show that the early dynamics of these two online movements follow the same mathematical order despite their stark ideological, geographical, and cultural differences. The evolution of both movements, across scales, follows a single shockwave equation that accounts for heterogeneity in online interactions. These scientific properties suggest specific policies to address online extremism and radicalization. We show how actions by social media platforms could disrupt the onset and ‘flatten the curve’ of such online extremism by nudging its collective chemistry. Our results provide a system-level understanding of the emergence of extremist movements that yields fresh insight into their evolution and possible interventions to limit their growth.

1D charge density wave in the hidden order state of URu2Si2

Single crystals of URu2Si2 show below 17.5 K a transition into an ordered state with a significantly reduced entropy. The low temperature phase is called the hidden order (HO) state, because its microscopic origin is still unknown—there are no charge, structural or magnetic transitions associated to HO. Here we report a one-dimensional (1D) charge modulation within the HO state of URu2Si2. We perform detailed Scanning Tunneling Microscopy (STM) experiments with high resolution on many atomically flat surfaces of URu2Si2 obtained by fracturing single crystals at cryogenic temperatures and find a 1D charge density wave with a large wavevector. We show that the 1D modulation is connected to the dynamical magnetic excitations of the HO through a moiré construction and appears as a consequence of excitations quenched through the interaction between the travelling fracture front and the dynamic modes of the crystal. The combination of fracture dynamics and the dynamics of the solid provides a method to create ground states and shows that charge interactions are among the most relevant features competing with HO in URu2Si2.

Isoelectronic perturbations to f-d-electron hybridization and the enhancement of hidden order in URu2Si2

Electrical resistivity measurements were performed on single crystals of URu2–xOsxSi2 up to x = 0.28 under hydrostatic pressure up to P = 2 GPa. As the Os concentration, x, is increased, 1) the lattice expands, creating an effective negative chemical pressure Pch(x); 2) the hidden-order (HO) phase is enhanced and the system is driven toward a large-moment antiferromagnetic (LMAFM) phase; and 3) less external pressure Pc is required to induce the HO→LMAFM phase transition. We compare the behavior of the T(x, P) phase boundary reported here for the URu2-xOsxSi2 system with previous reports of enhanced HO in URu2Si2 upon tuning with P or similarly in URu2–xFexSi2 upon tuning with positive Pch(x). It is noteworthy that pressure, Fe substitution, and Os substitution are the only known perturbations that enhance the HO phase and induce the first-order transition to the LMAFM phase in URu2Si2. We present a scenario in which the application of pressure or the isoelectronic substitution of Fe and Os ions for Ru results in an increase in the hybridization of the U-5f-electron and transition metal d-electron states which leads to electronic instability in the paramagnetic phase and the concurrent formation of HO (and LMAFM) in URu2Si2. Calculations in the tight-binding approximation are included to determine the strength of hybridization between the U-5f-electron states and the d-electron states of Ru and its isoelectronic Fe and Os substituents in URu2Si2.

Elements of expressionism in the story of A. Green «Gray Car»

The topic of the work is relevant, since at present the Russian literary criticism is rediscovering the features of the artistic expression of Soviet literature of the 1920s. The aim of the study was to discover the features of the poetics of Greens text that allow us to identify the novel under consideration as expressionist. Research methods used in the article: comparative-historical and semantic analysis of texts. At the beginning of the article, the research of the novella Gray Car by A.V. Polupanova and L.U. Zvonareva is considered, inaccuracies and obvious factual errors are noted, indicating a lack of understanding of the text, a mixture of such forms of expression of the authors position (according to B.O. Korman) as hero-narrator and author-narrator. As an empirical material, in addition to the novel by A. Green, the works of G. Mayrink, F. Kafka and L. Perutz were used. As a result of consideration of the novels the Golem, Castle, the Cossack and the Nightingale, the Wizard of judgment was allocated with the following features art in the world of the expressionist literary works: a) the problem of identity associated, as a rule, the main character; 2) semema madness, which became a constant for expressionist texts; 3) mysterious (hidden) order of a seemingly chaotic world; 4) the lack of success of speech acts. All these features were found in the story of A. Green, so it seems that the new approach to the work of the writer as a representative of Russian expressionism, demonstrates its consistency and can be contrasted with the more traditional consideration of the authors prose as a super-textual unity.

Hidden order and multipolar exchange striction in a correlated f-electron system

The nature of order in low-temperature phases of some materials is not directly seen by experiment. Such “hidden orders” (HOs) may inspire decades of research to identify the mechanism underlying those exotic states of matter. In insulators, HO phases originate in degenerate many-electron states on localized f or d shells that may harbor high-rank multipole moments. Coupled by intersite exchange, those moments form a vast space of competing order parameters. Here, we show how the ground-state order and magnetic excitations of a prototypical HO system, neptunium dioxide NpO2, can be fully described by a low-energy Hamiltonian derived by a many-body ab initio force theorem method. Superexchange interactions between the lowest crystal-field quadruplet of Np4+ ions induce a primary noncollinear order of time-odd rank 5 (triakontadipolar) moments with a secondary quadrupole order preserving the cubic symmetry of NpO2. Our study also reveals an unconventional multipolar exchange striction mechanism behind the anomalous volume contraction of the NpO2 HO phase.